The present invention relates to an ink jet printing apparatus which includes an ink jet head for ejecting ink under supersonic vibration from its nozzle, a charging electrode for selectively charging droplets of ink at a position where the droplets become separated successively from the stream of ejected ink, and deflection electrodes for deflecting charged ink droplets to cause them into impingement on a sheet of paper.
In an ink jet printer of the type described, ink in the nozzle of the ink jet head tends to set as after a long time of suspension of operation of the printer. Setting of ink in the nozzle results in deviation of the direction of ink ejection to an unusual or uncontrollable range when the printer is operated after the suspension. Another cause of such deviation in the direction of ink ejection is constituted by dust particles and the like which might be deposited on the wall of the nozzle. Once out of expected bearings, ink droplets would impinge on the charging electrode and/or deflection electrodes to invite various accidents such as electric discharge.
When a voltage pulse fails to be coupled to the charging electrode in properly timed relation to the formation of an ink droplet due to clogging of the nozzle, the ink droplet cannot be deposited with a desired amount of electrostatic charge and, therefore, prints a dot on the paper sheet in a position offset from expected one. Such dislocated ink droplets would disturb the reproduction of a desired image on the paper sheet. With this in view, it has been customary to determine a proper timing for applying a voltage pulse to the charging electrode after a search of a proper charging phase for an ink droplet. A critical drawback of this system is that the detection of an accidental change in the direction of ink ejection is impossible or, if not impossible, incomplete though satisfactory to determine whether the charge timing is proper.